Study On The Formation Mechanism And Simulation Prediction Of Defects In The Process Of Hot-press Forming For Aerospace Composites

There are a large number of special-shaped composite parts in civil aircraft.The traditional manual manufacturing method has many problems,such as low efficiency and unstable quality.The automatic hot press-forming technology can shape the prepreg into the required geometric preforming body with high quality and efficiency,and then send it to the autoclaved tank for curing.Compared with the hot diaphragm technology,the hot press-forming technology not only exert more pressure on the prepreg,but also control the pressure in different areas to improve the molding quality.In the process of hot press-forming,prepregs undergo complex deformation,which may result in intraply warp,wrinkle and other defects.In order to effectively predict and control the occurrence of defects and improve the quality of preforming,it is necessary to study the deformation mechanism.Therefore,the main purpose of this paper is to investigate the deformation mechanism of prepreg and the causes of defects,to explore the key factors in hot press-forming process,to establish the finite element simulation method,to achieve the prediction of preforming defects and then guide the optimization of hot press-forming process.In view of the above research objectives,the main research contents are as follows:Firstly,the deformation mechanism of prepreg and the causes of defects in the hot press-forming were studied by the theoretical analysis and experiment with the hatstringer.By analyzing the deformation of the prepreg,the mechanical behavior of the prepreg was investigated,and the important influence of the compression and inter-ply sliding behavior of the prepreg on the forming quality was determined.The molding quality under different conditions was characterized by the visual inspection and the metallographic detection.The impact mechanism of preforming parameters such as temperature and velocity were clarified,which provided theoretical and experimental basis for the simulation of hot press-forming process.Secondly,the compression response behavior of prepregs in the process of hot pressforming was investigated,and viscoelastic model was used to describe the compression and stress relaxation stage of prepregs.Combined with the theoretical analysis and experiment,the influences of compression distance,compression velocity,prepreg layers,the fiber orientation on the compression response behavior of prepreg were systematically studied.The viscoelastic model was used to fit the nonlinear compression response behavior of prepreg,and the applicability of different models was analyzed.With fewer parameters and clear physical meanings,Scott-Blair fractional viscoelastic model was used to describe the stress relaxation behavior of prepreg.The nonlinear fitting equation of the model parameters was established to predict the compression response behavior of prepreg at different temperatures and velocities.The results can be used to calculate the blank holder force in finite element simulation.Thirdly,the factors and mechanism of inter-ply sliding behavior of prepregs in the process of hot press-forming were investigated,and the prediction of inter-ply sliding resistance was realized.An inter-ply sliding resistance measuring device was designed.The influence of pressure,temperature,velocity and fiber orientation on the inter-ply sliding behavior of prepreg were explored.Through theoretical calculation and experimental verification,the lubrication state of resin during the sliding process was clarified.The mesoscopic mechanism of the inter-ply sliding was studied,and the prediction model of inter-ply sliding resistance considering pressure,velocity and fiber orientation was established.It can be used to calculate the friction in the process of hot press-forming.Finally,the finite element simulation model was established,which realized the defect prediction of the hot press-forming process.The successful preparation of the 8m hyperbolic hat-stringer verified the effectiveness of the method.In the finite element model,the behavior of prepreg was divided into the intra-ply and inter-ply deformation behavior.The intra-ply deformation behavior was decoupled into in-plane mechanical behavior represented by membrane element and out-plane bending behavior represented by shell element.The membrane element was modeled by the fiber tracking mode The shell element represented the flexural stiffness of the prepreg by the equivalent tensile modulus.The inter-ply sliding model was used to describe the interply sliding behavior of prepreg.The blank holder force under different conditions was calculated using the viscoelastic model of prepreg compression response.The hot pressforming process of the straight and hyperbolic hat-stringer was simulated by the finite element model,and the influence of blank holder force,temperature and velocity was explored.The accuracy of the model was verified by comparing simulation and experiment results.This model has guided the hot press-forming process of the 8m hyperbolic hat-stringer,which effectively controlled the forming quality.The product has no wrinkle defects,which shows the accuracy of the experimental method and modeling ideas.